In conventional temperature or climate control systems (e.g. heating, ventilating and air conditioning systems, referred to herein generally as "HVAC"), thermostats are used to control when the HVAC system turns on and off. The user presets a desired temperature (or "user set point"), and when the temperature of the controlled space is different from the preset temperature, the HVAC system heats or cools the air until the preset temperature is reached.
Thus, conventional enclosed space thermostats are merely on/off switches with a sensor to measure the enclosed space temperature and means for users to set their preferred temperatures. A problem with such thermostats is that the temperature is maintained at the user set point whether people are present or not, using costly natural resources. Heating or cooling when people are not present wastes a great deal of these resources.
Some enclosed space thermostats come with a built-in clock and have a method for people to program different user set points for different times and days. Such clock-thermostats provide different HVAC service when people are expected to be present than when they are expected to be absent. Problems with this approach are that the programming of the clock-thermostats is troublesome, and moreover that, even when the programming is done correctly, people's schedules change and thus often do not match the preprogrammed times.
Some enclosed space thermostats come with sensors to detect people. They switch from one conventional thermostat when people are present to a second conventional thermostat when people are absent. The second thermostat may have a second fixed temperature, in order that it may shift ("setback" or "setup") a fixed number of degrees from the first temperature when people are absent. The problem with these thermostats is the second temperature is often either too far away from the first temperature to provide satisfactory comfort when someone returns to the room, or too close to the first temperature to achieve adequate energy savings. Further, enclosed space and ambient conditions change constantly, so these thermostats are very difficult to regulate for optimal energy savings versus comfort trade-offs. Even if a user could figure out the optimum second thermostat setting for comfort and maximum energy savings at a given time, conditions constantly change and that setting may quickly become non-optimal. (In general, the term "ambient"--as in "ambient temperature"--will be used to refer to the temperature or other conditions of the region surrounding the controlled, usually enclosed, room or other space. "Space temperatures" will be used to refer to temperatures within the controlled space.)
There is thus a need for a climate control system that takes into account the occupancy status of a controlled space and automatically responds to variations in space and ambient conditions so as to minimize energy usage while meeting predetermined comfort, health and other criteria that may be preset by the user. Such a system should preferably take into account variable comfort settings for different people, both as to temperature and as to the time allowed for the temperature to recover to the preferred setting when people return from an absence (referred to herein as "recovery time"). The system should also automatically accommodate variable enclosed space conditions, including variable thermal energy leakage to and from the enclosed space, and variable thermal energy sinks (furniture, equipment, wall and floor coverings, etc.) in the enclosed space. Moreover, variable ambient conditions should be accommodated (such as day or night, summer or winter, clear or rainy, calm or windy), and additionally the system should compensate for variations in HVAC equipment operating capabilities. All of these goals are met automatically by the system of the present invention.
Conventional systems do not take into account the nonlinear relationships between HVAC equipment operation and the responses (temperature, humidity, etc.) of the controlled space with time. There is a need for a system that recognizes and utilizes such nonlinear relationships to effect climate control, such as by using exponential curve fitting.